1. Field of the Invention
The present invention relates to an overhead installed communication system, and, in particular, to an optical subscriber-line network unit installed overhead in the open air.
FIG. 1 generally shows a communication system to which optical subscriber-line network units are applied. This communication system has been proposed as a near feature system for the multimedia-oriented trend. In this communication system, an optical cable extends just in front of each house.
Telephone stations 10 and 11 are connected through an optical cable 12 with one another. A plurality of optical cables 13 extending from the telephone station 11 are laid between poles 14 and reach to just in front of houses 15, respectively. For example, one overhead installed optical subscriber-line network unit 20 is provided for each ten houses and is installed overhead in the open air. Each optical subscriber-line network unit 20 includes a light-electricity converter 21, a multiplexer and demultiplexer unit 22 and ten-odd subscriber channel units 23. An extending end of each optical cable 13 is connected to a respective optical subscriber-line network unit 20 and also a power line 17 is connected to the unit 20. Each optical subscriber-line network unit operates with power supplied through the power line 17. One external metal cable 18 extends from each subscriber channel unit 23, and thus, ten-odd external metal cables 18 extend from each optical subscriber-line network unit 20. Each external metal cable 18 extends into the respective house 15 and is connected to a communication terminal 19 for a telephone, a facsimile machine and so forth.
In this communication system, each optical cable 13 extends just in front of the houses and the length of each external metal cable 18 is short. Accordingly, it is possible to transmit a large amount of information in comparison to the prior art. Voice transmission, image transmission, digital signal transmission and so forth are performed.
Because each optical subscriber-line network unit 20 is installed overhead in the open air, it is desired that each unit 20 be miniaturized, lightweight, weatherproof, have superior reliability, and so forth.
2. Description of the Related Art
Recently, various optical subscriber-line network units installed overhead in the open air are being developed.
However, it has been difficult to develop an overhead installed optical subscriber-line network unit which is waterproof, has heat radiating performance and so forth in the condition where the unit is miniaturized and lightweight.